This invention relates to the preparation of polyethers having reduced levels of terminal unsaturation.
Polyethers are well known materials which are useful for, among other things, preparing polyurethanes. The polyethers predominantly used in making polyurethanes are polymers of alkylene oxides such as ethylene oxide, propylene oxide, butylene oxide, and mixtures thereof. The polyethers are commonly prepared by heating the alkylene oxide in the presence of an initiator compound as described later and a basic catalyst such as potassium hydroxide.
Under the conditions of the polymerization reaction, alkylene oxides, in particular propylene oxide, can generate an unsaturated alcohol (or the corresponding oxy anion). In the case of propylene oxide, allyloxy anion (or the corresponding alcohol) is formed. Allyloxy anion acts as a monofunctional initiator in the polymerization reaction, thereby forming monofunctional polyethers (referred to herein as "monols") having terminal unsaturation due to the residue of the unsaturated alcohol. Because of a rearrangement reaction, some of the terminal allyloxy (CH.sub.2 .dbd.CH--CH.sub.2 --O--) groups can isomerize to form a propenyloxy (CH.sub.3 --CH.dbd.CH--O--) group.
The problem of alkylene oxide conversion to the allyloxy anion increases substantially as the molecular weight of the polyether increases. Therefore, the amount of monofunctional species increases as the polyether molecular weight increases. Thus, the problem of monol formation is quite small in low equivalent weight polyethers, but as the equivalent weight of the polyether increases above about 500 or so, it starts to become significant. At equivalent weights above about 1000, the problem can be so significant that 10 to 50 mole % or more of the molecules in the polyether are monols.
The monols are undesirable because when a polyurethane is prepared from the polyether, the monofunctional polyethers terminate the polymer chains and thereby limit the molecular weight of the polyurethane. This has been related to certain shortcomings in the physical properties and processing characteristics of the polyurethanes. Thus, it is desirable to reduce the level of monols in the polyether as much as possible in order to improve the properties of polyurethanes made therefrom.
Several attempts have been made to reduce the monol content of polyether polyols. Several of these focus on reducing the amount of alkylene oxide conversion to allyloxy anion so that the monols do not form in the first place. These methods employ special catalysts such as barium hydroxide, or relatively mild reaction conditions. Each results in some decrease in monol formation, but not as much as desired. Moreover, the use of mild reaction conditions has a very substantial drawback in that it drastically slows the rate of polyether formation, and therefore a much longer reaction time is required to prepare the desired product. This has the effect of reducing the capacity of a polyether plant by up to 50% or more, and thus is very disadvantageous.
Other attempts to reduce the monol concentration have involved the elimination of the propenyl-type unsaturation (RCH.sub.2 --CH.dbd.CH--O--, wherein R is hydrogen or inertly substituted alkyl) by acid hydrolysis. This creates a lower aldehyde, (propionaldehyde in the usual case when R is hydrogen) and a polyether diol. This method is effective in reducing the propenyl unsaturation, but does not affect the allyl (RCH.dbd.CH--CH.sub.2 --O--, wherein R is as defined before) unsaturation, which normally constitutes about 70-90% of the unsaturation. Thus, this method does not provide a means to form a very low monol content polyether.
It would therefore be desirable to provide a process in which a polyol with a very low monol content is formed.